Many different mobile robots are known in the art such as autonomous mobile robots and conventional, operator-driven vehicles. Autonomous mobile robots typically have different mobility requirements than conventional, operator-driven vehicles. These differences are dictated by the need for simple control, and by the nature of the interior environments in which they must operate, typically offices and warehouses with narrow aisles. Many nuclear applications also have narrow aisles with tight corners and dead ends. In these environments, a vehicle must be able to turn within its own footprint and must have good odometry for measuring the distance traveled. If the vehicle's controlling computer cannot estimate its movement from its drive system encoders, it will require expensive gyroscopes and/or inertial sensors for navigation. The vehicle must also negotiate bumps and ramps, and operate without damaging floor surfaces.
During the early 1980s, as research into autonomous robots intensified, mobile robots having a synchronous drive train gained popularity, also known in the art as Synchro-drive. With Synchro-drive mobile robots, all wheels of the vehicle steer and drive synchronously. All wheels turn in unison and trace parallel, equal length paths to each other. The platform does not rotate as the wheels steer, so it remains in the same orientation regardless of its direction of movement. A turret flange at the top center of the mobile robot rotates in unison with the steering and accommodates a subturret that turns in the direction of the forward motion of the mobile robot. A Synchro-drive mobile robot can thus follow any path geometry.
Synchro-drive mobile robots have many advantages. For example, the most important advantages are: excellent odometry, excellent traction, zero turning radius and high maneuverability, low destructive forces on floor (when properly implemented), ease of control and efficiency. In mobile robots having Synchro-drive, all wheel driving forces are perpetually parallel, providing excellent traction and accurate measurement of relative motion (odometry). Because all wheels are driven by the same motor, the vehicle does not lose traction as its weight shifts from one wheel to another (a common failure of alternative designs). Most importantly, Synchro-drive exhibits far less coupling between driving and steering forces than other competitive techniques. This characteristic minimizes heading errors that can be induced by bumps and slippage. With steering and drive decoupled, control is a simple matter of applying polar geometry. Moreover, in mobile robots having Synchro-drive, the platform does not rotate as it executes a turn, so the angle between the turret and the base can serve as a relative heading reference. This effect, combined with the fact that all wheels move synchronously with respect to each other and must therefore execute equal length paths, causes the platform to drive a straight line with little arcing. The reason for this is that in order to arc (without steering movement), the wheels on the outside of the curve would be forced to travel further than those on the inside, and they cannot do so without skidding.
Finally, because there are extremely small counteracting forces in a Synchro-drive mobile robot, it does not waste precious battery energy fighting its own movement. This results in vehicles which can patrol continuously for as many as 24 hours. Since its introduction in the early 1980's, the Synchro-drive mobile robot has gained widening acceptance as an optimal mobility technology for mobile robots.
U.S. Pat. No. 4,573,548 is an example of a first generation Synchro-drive mobile robot which uses two horizontal belts or chains. As shown in FIG. 15 of U.S. Pat. No. 4,573,548, one chain couples the drive motor to the wheel assemblies which are spaced evenly around the base of the mobile robot, and a second chain couples the steering motor to the wheel assemblies. The first generation mobile robot was controlled by remote control. Because the drive chains introduced unacceptable odometry errors, the first generation machine was not controlled autonomously. The subject matter of U.S. Pat. No. 4,573,548 is assigned to the assignee of the instant application, and hereby incorporated by reference.
In other first generation machines, prototypes have also been constructed with pinion gears arranged in rows to carry driving and steering forces from a central mover to the wheels. These systems suffer from excessive accumulated backlash and have never become popular.
U.S. Pat. No. 4,657,104 is an example of a second generation Synchro-drive mobile robot. As shown in FIG. 27 of U.S. Pat. No. 4,657,104, the second generation mobile robot has a three-wheel Synchro-drive platform with a steering and drive shaft system which are concentrically arranged to replace the chains.
In the second generation Synchro-drive mobile robot, each wheel assembly has one wheel mounted to the side of its respective "foot", which is mechanically geared so that the wheel rolls around the foot during steering to avoid destructive and power consuming twisting under the center of the foot. The gearing therefore acts as a mechanical adder which provides the rotational drive to each wheel. This action is defined by the following equation of motion: .omega..sub.W =R(.omega..sub.D -.omega..sub.S) where:
.omega..sub.W =Angular velocity of wheel PA1 .omega..sub.D =Angular velocity of drive shaft PA1 .omega..sub.S =Angular velocity of steering PA1 R=r'/r=A/B PA1 r'=Wheel offset from steering pivot PA1 r=Radius of wheel PA1 A=No. of teeth on power shaft gear PA1 B=No. of teeth on wheel shaft gear
The design of the second generation Synchro-drive mobile robot causes the wheels to protrude from under the body at one extreme so as not to be dangerously close to the center of gravity at the opposite orientation. The result is that the second generation Synchro-drive mobile robot has a clearance (worst case) of 32.75 inches (83cm), allowing it to narrowly pass through a standard 36 inch door. When the second generation Synchro-drive mobile robot performs a tight maneuver such as going through a 36 inch door, it must carefully measure the door with its sonar, and then adjust its path laterally to compensate for its footprint (which it calculates from the angle between the base and turret).
Since 1984, over 60 of these vehicles have been placed in operation around the world in military, industrial, research, security, building monitoring, and nuclear applications. Since 1990, Synchro-drive mobile robots have been in routine commercial service in security, material handling and nuclear applications. The subject matter of U.S. Pat. No. 4,657,104 is assigned to the assignee of the instant application, and hereby incorporated by reference. The second generation mobile robots has been sold under the name Navmaster.TM. and Cyber-Guard.TM. by Cybermotion.
One disadvantage of the second generation Synchro-drive mobile robot is that the forces around the base and around each foot are not perfectly symmetrical. When the mobile robot accelerates or decelerates, an unbalanced torque force is placed on the steering axis. The imbalance dictates that the steering system backlash be very small to maintain good odometry.
Despite its wide spread acceptance, the second generation Synchro-drive mobile robot has another disadvantage of being unable to control the base orientation. This fact, combined with the asymmetry of a three-wheeled platform, means that in any direction of movement, a wheel assembly may be straight out to one side (the worst case for lateral clearance) or straight out to the other side, or at any position in between.